1. Field of the Invention
This invention relates to a system and a technique for identifying discontinuities, manufacturing flaws or other faults along the length of an optical fiber of the kind used for information or data transmission. In particular, the invention concerns an arrangement for determining performance characteristics of multimode optical fibers (MMFs).
2. Discussion of the Known Art
Multimode optical fibers typically have cores whose index of refraction (RI) is either constant throughout the core, or whose RI gradually diminishes radially outward from the core axis to a value that approaches the RI of a surrounding cladding. The former are referred to as step-index MMFs while the latter are known as graded-index fibers. Also, the cores of typical MMFs have diameters substantially greater than those of cores in single mode fibers (SMFs), so that a source of light for a MMF need not have as great an intensity as would be required for operation with a SMF. Typical core/cladding diameter ratios are 100 μm/140 μm and 200 μm/240 μm, for step index MMFs; and 50 μm/125 μm and 62.5 μm/125 μm, for graded index MMFs. By contrast, a typical SMF has a core/cladding diameter ratio of, for example, only 9 μm/125 μm. See J. Hecht, Understanding Fiber Optics, pages 55-72, Prentice Hall (3d ed. 1999)(“Hecht”), which is incorporated by reference.
U.S. Pat. No. 4,286,979 (Sep. 1, 1981) is directed to a method of producing MMFs having improved dispersion characteristics. Specifically, light pulses from a laser source are launched into a first end face of a MMF through a singlemode fiber, by positioning an output end face of the SMF at a radial position on the core of the MMF that corresponds to a certain mode subgroup supported by the MMF. Such positioning is carried out with commercially available micropositioners using, e.g., adjustable micromanipulator vacuum chucks wherein the output end face of the SMF and the first end face of the MMF are retained with an axial gap of, e.g., less than 10 μm between the end faces and with the fiber axes parallel to one another. A refractive index matching oil or fluid fills the gap, and the chucks are adjustably displaced relative to one another in a radial direction with a resolution on the order of about 0.1 μm.
Light pulses output from a distal, second end face of the MMF are detected by a photodiode and differences in the timing of peaks in the output light pulses for each mode group are measured to obtain a differential mode group delay fiber characteristic. If the measurement results are not satisfactory for a particular fiber application, the results may then be used to modify accordingly the process by which preforms are being made in the production of the MMF. See also, U.S. Pat. No. 6,400,450 (Jun. 4, 2002) which discloses a method of qualifying a multimode optical fiber for bandwidth performance using a test set-up similar to that disclosed in the '979 patent. Both of the '979 and the '450 U.S. Patents are incorporated by reference.
Because it is not always practicable to perform end-to-end testing of an optical fiber which in typical installations extends over a distance on the order of kilometers, optical time domain reflectometers (OTDRs) which need to be coupled only to one end of a fiber under test, are now popular as a means to evaluate fiber performance. Losses, faults, reflections and other discontinuities, all of which are commonly referred to as “events”, can be indicated on a display stage of the OTDR.
Specifically, OTDRs detect light that is backscattered within an optical fiber in response to light pulses that are launched with preset durations and frequency from an operating port of the OTDR into the one end of the fiber under test. The time delay and relative amplitude of the detected backscattered light is displayed as a function of distance along the fiber. See, A. H. Cherin, “An Introduction to Optical Fibers”, Bell Laboratories, at pages 199-201 (1983); J. J. Refi, “Fiber Optic Cable—A LightGuide”, AT&T Bell Laboratories Specialized Series, abc TeleTraining, Inc., pages 156-63 (1991); Agilent Technologies (Germany) GmbH, Optical Time Domain Reflectometers—Pocket Guide (2001), at pages 13-21; Hecht, at pages 361-363; and Optronics EYT, Tutorials—OTDRs, at Internet (web) address <http://www.optronics.gr> (2003)(“Optronics”); all of which are incorporated by reference.
While OTDRs are constructed and used mainly for testing performance of long haul SMFs, configurations have been disclosed wherein OTDRs are used to measure performance characteristics of multimode fibers that span only hundreds rather than thousands of meters. Such MMFs are frequently encountered in office buildings, campuses, and local area networks (LANs). See Optronics, supra, at pages 8-9. Further, U.S. Pat. No. 6,421,117 (Jul. 16, 2002) discloses apparatus for performing time domain reflectometry on a multi-mode optical fiber, wherein a light source in an OTDR includes a laser diode, and a lens that focuses light from the diode onto a core of a light source MMF at a position offset a certain distance in a normal direction from the central axis of the light source fiber. Backscatter light is produced more uniformly over the length of the fiber under test for detection by the OTDR to enable more accurate measurements to be performed, according to the patent.
As far as is known, OTDRs have not been applied to determine or to obtain a transmission profile of a multimode fiber at a given location over a span of the fiber. Typically, such a measurement involves cutting the fiber and/or removing a protective coating that surrounds the fiber cladding at the given location. A known refractive near field method of measuring the transmission profile requires that the fiber be broken at the given location in order to obtain the profile at that location. Moreover, an interference method of measuring the profile also requires the removal of a protective plastic coating on the fiber cladding.
Uniformity of the entire light transmission profile along a MMF is essential in applications involving transmission data rates at 10 Gbps or higher. Suppliers of MMF for such high bandwidth applications must ensure fiber uniformity to their customers. Moreover, the ability to locate and identify defects precisely in a multimode fiber can serve as a diagnostic tool in fiber production, as well as a means for evaluating the quality of splices at various known locations over the length of an installed fiber. A system and technique that can determine attenuation or loss within a multimode fiber as a function of distance as well as radial position in the fiber, mode groups, and any changes in attenuation caused by point discontinuities, would therefore be highly desirable.